I think Meridian's EZ in 7.1 uses the 2 rears the way 6.1 EX would use the center speaker.

Using steering type of rear surround, any sound source that pans from left to right would go from 2 speaker mode (balance between the L&R speaker with emphasis on one of the channel,) to center speaker sound only, then to 2 speaker mode. Causing a unatural shift in sound, which is the hallmark of DPL.

Trifield will only enhance the playback of the sound. Unless it is EX encoded, where they have intentionally put the rear center sound and expect you to only hear that single rear cente speaker (like that little ballon man in the THX logo demo going to the center rear and doing his thing.) Besides that, anything that involves sound from both L&R surround, Trifield mode would be better.